Swine flu has swept through about 22 million Americans from April to October, killing an estimated 3,900 people, including 540 children, health officials said Thursday.

The analysis represents the government's latest effort to assess a viral outbreak that in just six months has flooded emergency rooms and intensive-care beds in at least 48 states that have reported widespread flu cases. With flu season just beginning, an estimated 98,000 people have been hospitalized, according to the Centers for Disease Control and Prevention (CDC).

"We've been tracking influenza for decades," says Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases. "What we are seeing in 2009 is unprecedented."

The new national estimates are extrapolations of data drawn from a CDC emerging-diseases network of hospitals, laboratories and health departments in 10 states and from reports of hospitalizations and deaths, Schuchat says. They illustrate the extent to which swine flu, also known as H1N1, is hitting children, who account for 8 million infections, 36,000 hospitalizations and 540 deaths. In a typical flu season, about 80 children die.

Arnold Monto, a flu expert at the University of Michigan, says the government must rely on estimates such as these because flu tests in doctors' offices are unreliable and most diagnoses are based on patients' symptoms. That makes flu reporting sketchy at best.

"We know we are missing cases," Monto says. "What they're trying to do with these estimation methods is fill in the gaps."

Fortunately, most flu cases so far have run their course after a few days of bed rest, fluids and aspirin or acetaminophen to combat fever, doctors say. But the disease has taken a disproportionate toll among pregnant women, diabetics and young people, especially those with neuromuscular ailments such as cerebral palsy and muscular dystrophy.

In a typical flu season, which ends in May, 90% of deaths and hospitalizations involve people 65 and older. The H1N1 outbreak began in April; 90% of those who become infected and need medical care are younger than 65.

Schuchat urges people who appear to be getting severely ill and those with chronic conditions and asthma to quickly see a doctor. Antiviral drugs, such as Tamiflu and Relenza, are most effective when given soon after symptoms set in, she says. What worries her most is that flu season has barely begun: "We have a long flu season ahead of us."